This invention relates to an ion source for an ion implanter, an ion milling machine or the like, and more particularly to an ion source suitable for obtaining a stable ion beam for an extended period of time, and further particularly to a microwave ion source suitable for obtaining a B.sup.+ ion beam.
FIG. 1 is a schematic illustration of the structure of a conventional microwave ion source.
The microwave ion source consists of rectangular waveguides 2a and 2b as the waveguide for propagating a microwave, discharge electrodes 4 constituting a ridged waveguide structure, a discharge chamber 5 made of boron nitride and disposed between the ridged waveguides, and extraction electrodes 8a, 8b and 8c which extract the ion beam 21. An axial magnetic field generated by the excitation of a coil 13 is applied to the discharge chamber 5, and a feed gas for discharge is introduced into the discharge chamber 5 through a gas introduction pipe 6.
FIG. 2 shows in detail the discharge chamber and portions near the chamber. FIG. 3 is a sectional view of the discharge chamber 5 and portions near the chamber, and is useful for explaining the discharge chamber 5, a gas inlet 10 and an ion beam exit slit 7. In the drawings, reference numeral 1 represents a microwave generator; 3 is a microwave introducing flange; 5a is a lining of the discharge chamber 5; 7a is a portion near the ion beam exit slit; 11 is a dielectric filler; and 12 is an insulator.
PH.sub.3 (phosphine), AsH.sub.3 (arsine) or the like as a hydride is used as the feed gas in order to obtain the beam of P.sup.+ (phosphorus) ion, As.sup.+ (arsenic) ion beam or the like that is used for the ion implantation for a semiconductor in the ion source shown in FIG. 1. In this case, the P.sup.+ or As.sup.+ ion beam can be extracted stably for an extended period of time. If a BF.sub.3 gas is introduced in order to obtain B.sup.+ ion beam necessary for the ion implantation for a semiconductor, however, two problems develop, which have made it difficult so far to obtain stably a high current ion beam for an extended period of time:
(1) deposit at the ion beam extraction opening portion (i.e., ion beam exit slit 7); and PA0 (2) deposit inside the discharge chamber 5.
If the deposit of the item (1) occurs, the opening area is reduced so that the extracted beam current drops. In a conventional ion source, the gas inlet 10 is positioned near the center of the discharge chamber 5. Therefore, if plasma is generated by a halide gas such as BF.sub.3, boron nitride (BN) constituting the lining 5a of the discharge chamber 5, particularly its portion near the gas inlet 10, is etched, and an etching product (most of which is BN) precipitates at the other portions, particularly at the ion beam exit slit 7. In consequence, the slit width is reduced, and the current drops eventually. According to an experiment which uses BF.sub.3 gas as the feed gas, the area of the opening portion is reduced almost by half in the course of the operation of the ion source for about four hours. When the deposit is hit by the plasma or ion beam, it is peeled off from the ion beam exit slit 7 and flies sometimes into the space to which an electric field for extracting the ions is applied. The peeled matter strikes the electrode 8b and generates secondary electron emission, which in turn generates an abnormal discharge between the electrode 8a to which a high positive voltage is applied and the electrode 8b to which a high negative voltage is applied. This results in instability of the ion source.
The deposit is often peeled off in the case of (2), too, and the plasma state becomes unstable; and, in addition, an abnormal discharge develops between the electrodes in the same way as in the case of (1), and reduces the stability of the ion source.
If the plasma is generated in a conventional ion source with a halide gas such as BF.sub.3, the extracted ion beam current decreases and the stability of the ion source drops, as described above.
It is believed that the reason why the deposit occurs when, e.g., BF.sub.3 or BCl.sub.3 gas is used is that since the fluorine or chlorine atom generated by microwave discharge is extremely active chemically, it corrodes and dissociates boron nitride (BN) that constitutes the lining 5a of the discharge chamber 5. As a matter of fact, when the deposit is physically analyzed, it is determined to be boron nitride. In order to prevent the occurrence of such a deposit, it is effective to constitute the discharge chamber in a thermally isolated structure, and to raise its operation temperature so as to thermally dissociate or evaporate the deposit. However, the temperature cannot be raised beyond a certain limit due to the structural limitations of the discharge chamber (generally, the approximate upper limit is 800.degree. to 900.degree. C.), and it has been difficult in practice in the past to restrict the quantity of the deposit to a level which presents no practical problems.
As a prior art reference disclosing the state of the art in this field, mention can be made of Japanese Patent Laid-Open No. 132754/1981.